Author: (Poey, 1868)
Field Marks:
A big, stocky, short-nosed, pale yellow-brown requiem shark with the second dorsal about as large as the first, and narrow, smooth-cusped teeth in both jaws. Closely similar to the allopatric N. acutidens, but usually has less falcate fins.
Diagnostic Features:
Dorsal, pectoral and pelvic fins usually weakly falcate. Serrations well-developed on blades of upper teeth in individuals 1.4 m long and larger, and absent in small individuals 0.7 m or less. Total vertebral counts 197 to 206.
Geographical Distribution:
Western Atlantic: New Jerscy to southern Brazil, including Gulf of Mexico, Bahamas and Caribbean. Eastern North Atlantic: Senegal, Ivory Coast (probably wide-ranging off West Africa). Eastern Pacific: Southern Baja California and Gulf of California to Ecuador.
Habitat and Biology:
An abundant, coastal, inshore tropical shark of the continental and insular shelves, found from the surface and intertidal down to at least 92 m; it also occasionally ventures into the open ocean, near or at the surface, apparently for purposes of migration. It commonly occurs around coral keys, at mangrove fringes, around docks, on sand or coral mud bottoms, in saline creeks, in enclosed sounds or bays, and in river mouths. It may enter fresh water but does not penetrate far up tropical rivers as does Carcharhinus leucas. In the western North Atlantic lemon sharks are thought to be divided up into a Caribbean principal population and a Gulf of Mexico-Atlantic USA accessory population, though the principal population may use Florida as part of its nursery grounds. Individuals occur singly or form loose aggregations of up to 20 individuals, with some segregation by size and sex. Off Florida adult lemon sharks may migrate southward and into deeper water, at least in transit, at the onset of winter.
This shark is currently the subject of an intensive long-term behavioural and ecological study by Dr S.H. Gruber and associates. So far their work with sonic-tagged lemon sharks indicates that this species is active both day and night (with an average speed of slightly over 1.5 kph), but like some terrestrial predators is most active at dawn and dusk (shown by a 'speedup' to nearly 2.5 kph at these times). The lemon shark shows definite site specificity, especially in the young but to a lesser extent in adults, with a tendency in some individuals to return to the same favoured place each day. Lemon sharks tend not to passively drift with a current while moving and have been observed swimming in a set course which sometimes placed them against or across a current during a change in tidal direction. Although this species favours shallow areas, it readily can move into deeper water; one was observed to move from a reef into the Gulf Stream current and travel 100 km before returning to the shallows. With growth the sharks expand their home ranges dramatically, although still favouring shallow areas. Young sharks range over a limited space of 6 to 8 km on eelgrass flats, lagoons, and other shallow areas but as they grow to subadults gradually expand their range to about 300 km; adults additionally occupy offshore reefs and deeper water, especially for migrations, although they readily return to the shallows.
Studies of the respiratory physiology of lemon sharks suggest that they are adapted to being active in environments with a low oxygen level, such as the waters around mangrove bays which have high temperatures and high organic content. They have circulatory and respiratory mechanisms, such as blood with an unusually high affinity for oxygen, that enhances oxygen uptake. Lemon sharks are quite capable of resting on the bottom, but use up more energy at rest than when swimming at a normal rate; probably because of increased effort in pumping their gills when resting and from decreased efficiency of oxygen uptake with lower intake velocity of water through their gills. Comparative data on oxygen consumption show that this tropical shark operates at a metabolic level some 2+ times greater than the temperate piked dogfish (Squalus acanthias).
Viviparous, with a yolk-sac placenta; number of young 4 to 17 per litter. In the western North Atlantic, mating and birth occurs in spring and summer (May to September), with a peak in spring. The gestation period is between 10 and 12 months. Off the west coast of Florida there is a seasonal increase in abundance of these sharks as males and nongravid females congregate to mate. Pregnant females enter shallow nursery areas and drop their young, which stay there for considerable lengths of time. At one time this shark was thought to be fast-growing and to reach maturity in only one to two years (Springer, 1950b), but field studies by Dr Gruber and associates have revised this to about 6+ years. A growth curve published by Gruber (1981) gives an asymptotic maximum age of about 27 years. Growth of tagged free-ranging sharks is approximately 1/4 as great as those kept in captivity and given food to satiation, suggesting that availability of food is a limiting factor in the growth and ultimately reproduction of these sharks; as well as indicating that captive studies of the growth of sharks under ideal conditions may give an incorrect impression of what occurs in free-living sharks. Courtship behaviour and copulation has occurred in captivity, and wild-caught adult female sharks have scars from bites delivered above the pelvic fins by courting adult males. Lemon sharks do very well in captivity, with young individuals being favourite subjects for physiological and behavioural studies.
The lemon shark feeds primarily on fishes but also takes crustaceans and molluscs. Prey taken includes sea catfishes, mullet, jacks, croakers, porcupine fishes, cowfish (Ostraciontidae), guitarfish, stingrays, eagle rays (Pteromylaeus), crabs, crayfish, occasional sea birds, barnacles, amphipods, and conchs. Experimental studies on captive lemon sharks by Dr Gruber and associates showed that these sharks feed to satiation and stop, that young sharks 70 cm long eat 3% of their body weight daily with unlimited food available, and that they double their weight in about 100 days. With sharks that had been starved for three days, feeding them a meal equal in weight to what they normally chose in captivity (3% body weight) resulted in almost all food being digested in their stomachs after a day, but that allowing them to feed up to 20% of their body weight resulted in undigested food being retained in some cases for over two days. Meal size apparently is important in determining the rate of digestion and perhaps the feeding frequency, as well as the rate of growth.
The lemon shark has been involved in several attacks on people on boats, often after being disturbed, hooked or harpooned. There have been some unprovoked attacks by these sharks on bathers and swimmers, but more commonly the sharks are accosted by divers or anglers, which may result in the release of a vigorous attack. Although lemon sharks are apparently not aggressive to divers when undisturbed, and do not include mammalian prey as a significant part of their diet, they should be regarded as potentially quite dangerous because of their size, powerful jaws and large teeth, and tendency to defend themselves when disturbed; and should be treated with due respect. Baiting with fish underwater or spearfishing may result in close approaches by these sharks.
Size:
Maximum about 340 cm, males maturing at about 224 cm and reaching at least 279 cm, females maturing at about 239 cm and reaching at least 285 cm; size at birth 60 to 65 cm.
Interest to Fisheries:
A common inshore shark widely caught where it occurs, on longlines, and probably other gear; meat utilized dried salted, smoked, and probably fresh frozen, hides for leather and other products, fins for shark-fin soup base, oil extracted from the liver for vitamins, and carcasses for fishmeal.
Remarks:
Following Compagno (1979), the eastern Pacific N. fronto is synonymized with this species. The identification of the eastern Atlantic Negaprion with this species requires confirmation.
Type material:
Holotype: ? Type Locality: Cuba.